Global Quiz - world knowledge mapping projecthttp://en.globalquiz.orgGlobal Quiz is an addictive trivia game composed of difficult, ambitious questions.
However, behind the game, Global Quiz is also a research project which uses the database of selected answers to create the knowledge maps.

Tournament of Mindshttp://www.tom.edu.au/Tournament of Minds is a problem solving program for teams of students from both primary and secondary years. They are required to solve demanding, open-ended challenges from one of the following disciplines: Language Literature, Maths, Engineering and Social Sciences.
Tournament of Minds has been one of the fastest growing national interschool programs to challenge the youth of Australia and is now expanding internationally.

Activities for ESL Studentshttp://a4esl.org/Thousands of free-to-use online quizzes and activities for studying English as a second language.

Quiz Hubhttp://quizhub.com/quiz/quizhub.cfmQuiz Hub - Some Free but many on subscription [but no advertising as a result]... it is a collection of high-quality, interactive learning quizzes that help K-12 students enhance their core knowledge of English, French, German, Italian, Spanish, geography, government, history, art, music, mathematics, earth science, biology, chemistry, and physics.
The Quiz Hub also promotes analytical thinking, memory, and typing skills.

It Pays to Learnhttp://www.itpaystolearn.com/default.asp?ref_id=AGW500It Pays to Learn - lots of quizzes but you need to register first ... it's free but you do get lots-of adverts so be warned ... although they do appear to be safe ones ... they also have a system where you earn money credits for each quiz you take.

BrainPoP http://www.brainpop.com/BrainPoP - features Science, Biology and Technology subjects that correspond to the National Science Education Standards for 5-8 grades. Each subject contains a 3-4 minute Animated Movie, an Interactive Quiz, an Experiment, a Comic Strip, a How-to hands on application, a Timeline and a print-able Activity Page.